import java.util.*;
import java.io.*;

//Find the first non-repeated character in a string. If there are no such chars then return -1.
public class FirstNonRepeatingChar
{
	//Using Hash Map
	//Scanning once
	//Scanning twice

	public int findFirstNonRepeatingChar(String inputString)
	{
		char[] inputStringArray = inputString.toCharArray();
		int[] countArray = new int[26];
		int index = -1;


		for(int i = 0; i < inputString.length(); i++)
		{
			int tempAscii = (int) inputStringArray[i];
			countArray[tempAscii-97]+=1;
		}

		int i = 0;

		while(index == -1 && i < 26)
		{
			if(countArray[i]==1)
			{
				index = i;
			}

			i+=1;
		}

		return index;
	}

	public static void main(String[] args) {
		FirstNonRepeatingChar x = new FirstNonRepeatingChar();
		String inputString= new String(); 
		Scanner input = new Scanner(System.in);
		inputString = input.next();
		System.out.println("-----------------------------------------");
		System.out.println((char)(97+x.findFirstNonRepeatingChar(inputString)));	
		
		
		/* 
		 * Read From file using Scanner
		try {
			Scanner fin = new Scanner(new File("Input.txt"));
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		*/
		
		//	Scanner input = new Scanner(System.in);// Initializing a Scanner object 
		//inputString = input.nextLine();
	}


}